Grass mowing machines and similar vehicles may have hydraulic traction drive systems designed for improved traction of the machine. Several hydraulic traction drive systems are intended to prevent or limit wheel slip and optimize the machine's traction and hill climbing capability, while maintaining its performance and functionality.
For example, one type of hydraulic system intended for improved traction includes valving to control the flow and/or pressure being distributed to each of the wheel motors. U.S. Pat. No. 6,073,716 relates to such a hydraulic traction drive system having velocity fuses to stop the flow of hydraulic fluid to the slipping wheel. Such a system has advantages, but may cause instability in the system because the valves open and close flow passages in an on/off fashion. By relying on the flow induced pressure signals, such a system would not perform adequately under low flow conditions. Additionally, in rear steered vehicles, the vehicle dynamics in turns can cause false wheel slip signals to the fuses, causing unwanted reactions, especially if flow is curtailed to the outside wheel. While relying on the pressure differential across the valve caused by flow, there could be no signal on the wheel motor side of the valve to cause the valve to reopen allowing the wheel motor to regain drive.
Alternatively, another type of hydraulic system having a valving arrangement intended to improve traction includes speed sensors at the wheel motors, and proportional valves to meter the flow to the individual motors. When sudden wheel speed change is sensed, the valve meters the hydraulic flow to the respective wheel. However, the cost of such a system requiring sensors, valves and a controller is not feasible for many applications.
Another hydraulic traction drive system includes a flow divider or flow regulator to divert hydraulic flow away from a slipping wheel. Unfortunately, on front wheel drive machines, during a turn, the flow divider or flow regulator may divert flow away from the outside wheel, causing that wheel to stall which may prevent the machine from turning effectively. This type of hydraulic circuit may be used feasibly as an on demand differential lock activated by the operator. However, flow dividers also may be relatively high generators of heat within a hydraulic system, which is a nondesireable condition.
Another hydraulic traction drive system to limit wheel slip involves configuring the wheel motors into a parallel-series circuit. For example, U.S. Pat. No. 5,915,496 relates to a parallel-series four wheel drive hydraulic circuit for a riding lawn mower. However, a parallel-series circuit will not work effectively on a two-wheel drive machine. Also, in four-wheel drive vehicles, especially on front axle driven machines, a parallel-series system requires more horsepower to climb a hill when compared to a parallel circuit. This is due to the inherent sizing of the wheel motor displacements and the limited pressure to the second motor in series due to the pressure drop across the first motor.